During the last twenty years, sulfite pulp has increasingly been prepared by using soluble bases. Of the bases, magnesium base have led to the simplest and cheapest recovery systems. Therefore, processes using magnesium base, i.e., the magnesium sulfite processes, now dominate this portion of the industry.
The shift to magnesium base has in most cases solved the immediate environmental problems. The requirements on environmental protection, however, have become more stringent, and further measures are expected to be necessary in the future for reducing the effluent loads of substances from sulfite mills detrimental to the environment.
One of the greatest sources of effluent loads from a magnesium sulfite process is from the bleaching portion of the plant. One way of avoiding some of these effluent loads would be to use an oxygen bleaching stage. It is known that such an oxygen bleaching stage can produce sulfite pulp with low kappa numbers and can substantially reduce the effluent loads from the bleaching portion of the plant, e.g., BOD, COD and colour effluent load can be reduced.
Such an oxygen bleaching stage was, however, thought to have certain insurmountable problems in practice. Oxygen bleaching must take place in an alkaline atmosphere. Normally, sodium hydroxide is added in the oxygen bleaching for obtaining the alkaline conditions. Thus, the spent liquor from the oxygen bleaching will contain sodium. It is the presence of this sodium in the oxygen bleaching spent liquor which was believed to render oxygen bleaching impossible in practice.
In the normal process of producing magnesium sulfite pulp, the spent liquor from the digestion is combusted and the magnesium therein is recovered in the form of a magnesium oxide dust. This dust is then used in preparing new acid cooking liquor (for the digestion stage of the process).
In order to obtain the environmental advantages of oxygen bleaching, the spent liquor from the oxygen bleaching step also has to be recovered, evaporated and combusted. Since it would be much too expensive to evaporate and combust the oxygen spent liquor and the digestion spent liquor separately, the evaporation and combustion has to be carried out together to make the process economically feasible.
When spent liquors from pulping processes on sodium base are combusted, a smelt of sodium carbonate and sodium sulfide is obtained. Thus, it was believed that, upon combustion of spent liquor containing both magnesium and sodium compounds, the smelt formed by the sodium compounds would precipitate on the heat transfer surfaces in the boiler of the recovery system giving rise to insurmountable problems in the recovery of the magnesium oxide dust. In view of these problems, it was thought that oxygen bleaching of magnesium sulfite pulp was economically infeasible.
It is possible theoretically to use magnesium oxide as the alkaline source in the oxygen bleaching stage to try to avoid the problem. However, magnesium oxide is not suitable in view of some problems involved with it. For example, the delignification of the pulp to a desirable low kappa number cannot be carried out without unfavourably effecting the pulp quality. Satisfactory oxygen bleaching, therefore, in practice requires the use of sodium hydroxide as the alkaline source.